1. Field of the Invention
In one aspect, the present invention relates to methods for reforming hydrocarbons in the presence of steam. In another aspect, the present invention relates to apparatus for reforming hydrocarbons in the presence of steam.
2. Background of the Invention
As used in the art, the term "hydrocarbon reforming" generally refers to a catalytic process for dehydrogenating, dehydrocyclizing, aromatizing, and/or isomerizing a hydrocarbon feedstock. Typical reformer feedstocks include alkanes, cycloalkanes, and/or arylalkanes, each having up to 12 carbon atoms, and petroleum fractions such as straight-run naphthas, hydrocracked naphthas, thermally cracked naphthas, catalytically cracked naphthas, and the like. Hydrocarbon reforming can be used, for example, to: produce useful aromatic compounds (e.g., benzene and toluene), olefins, and/or isomers; upgrade the octane ratings of light gasoline fractions; produce highly aromatic aviation blending stocks; and provide substantial quantities of hydrogen which can be used elsewhere in the refinery or chemical plant (e.g., in a hydrodesulfurization unit). Various types of reforming processes are conducted in the presence of steam using steam-active reforming catalysts.
During the course of a steam-active reforming process, the steam-active reforming catalyst will become deactivated due to the deposition of carbonaceous materials on the catalyst's surfaces. These carbonaceous materials will typically consist of coke and/or polymeric substances. In order to remove the carbonaceous materials and thus reactivate the steam-active catalyst, the catalyst must be regenerated.
Depending primarily on the precise nature of the catalyst being used, steam-active reforming catalysts are typically regenerated using either a steam regeneration medium or a steam-diluted oxygen (or air) regeneration medium. When a steam regeneration medium is used, the regeneration steam reacts with the deactivating carbonaceous material to produce hydrogen and carbon monoxide. When a steam-diluted oxygen regeneration medium is used, the oxygen reacts with (i.e., combusts) the deactivating material to produce carbon dioxide and water.
Steam-active reforming processes provide several advantages. For example, the presence of dilution steam during the reforming operation serves to: (1) reduce the partial pressure of the hydrocarbon feedstock and thus provide improved conversion and product selectivity; (2) provide heat for, and moderate temperature losses resulting from, endothermic dehydrogenation and/or dehydrocyclization reactions occurring during the reforming operation; and (3) reduce the rate at which deactivating materials deposit on the catalyst,s surfaces. Additionally, when the catalyst is regenerated using a steam-diluted oxygen (or air) regeneration medium, the regeneration dilution steam absorbs the heat generated by the combustion of the deactivating material and thus serves to moderate temperature increases during the regeneration operation.
U.S. Pat. No. 2,906,696 discloses a process for reforming naphtha using a continuously regenerated activated carbon catalyst. The process of U.S. Pat. No. 2,906,696 is conducted using a fluidized catalyst reactor system. In order to remove carbonaceous deposits from the catalyst,s surfaces without consuming the catalyst itself, the regeneration process of U.S. Pat. No. 2,906,696 must be conducted at high temperature (i.e., 1600.degree.-2200.degree. F.) using a steam regeneration medium. Effluent gases produced during the steam regeneration process are circulated through the hydrocarbon reforming portion of the reactor system to provide a hydrogen-rich environment for the reforming operation.
U.S. Pat. No. 4,613,715 discloses a process for dehydrocyclizing C.sub.6 -C.sub.12 alkanes, naphthas, and/or synthetic gasolines in the presence of steam. The steam-active catalyst used in the process of U.S. Pat. No. 4,613,715 consists of a Group II metal aluminate and a Group VIII metal. During the endothermic dehydrocyclization process, reaction temperatures are maintained by injecting oxygen or air into the reaction system. The injection of oxygen generates heat by the combustion of a small amount of feed, hydrogen, and/or coke. The catalyst used in the process of U.S. Pat. No. 4,613,715 is regenerated by stopping the flow of hydrocarbon feed and treating the catalyst with steam diluted air.
U.S. Pat. No. 4,229,609 discloses a process for dehydrogenating a hydrocarbon feedstock using multiple, cyclically operated beds of a steam-active dehydrogenation catalyst. In the process of U.S. Pat. No. 4,229,609, the dehydrogenation reaction is conducted in one or more of the catalyst beds while the remaining catalyst beds are being regenerated. The regeneration of each catalyst bed is accomplished by stopping the flow of hydrocarbon feed thereto and then treating the bed with steam diluted oxygen. The gaseous effluent produced in the regeneration process can be used to indirectly heat the hydrocarbon feedstock. The steam-active catalyst used in the process of U.S. Pat. No. 4,229,609 is composed of: (1) a support select from the group consisting of alumina, silica, magnesia, zirconia, alumina-silicates, Group II aluminate spinels, and mixtures thereof; (2) a catalytically effective amount of at least one Group VIII metal; and, optionally, (3) at least one copromoter metal selected from lead, tin, and germanium.